There are known conventional semiconductor devices made of silicon, such as a trench-type Si-MOSFET. Japanese Patent Laid-Open No. 06-132539 discloses a semiconductor device having a vertical insulated gate field effect transistor that includes a semiconductor substrate of a first conductivity type, a first semiconductor layer of the first conductivity type of low impurity concentration formed on the principal surface of the semiconductor substrate, a second semiconductor layer of a second conductivity type formed on the upper surface of the first semiconductor layer, a third semiconductor layer of the first conductivity type formed on a part of a surface part of the second semiconductor layer, a gate oxide film formed on an inner wall surface of a gate trench, which has a substantially U-shaped cross section and extends from the surface of a central part of the third semiconductor layer through the part of the second semiconductor layer into the first semiconductor layer, a gate electrode formed to fill a groove on the gate oxide film, an insulating film formed to cover the gate electrode and an exposed surface of the second semiconductor layer, a gate wire that is formed on the insulating film and is in contact with the gate electrode, a source electrode that is in contact with the third semiconductor layer through a contact hole formed in the insulating film, and a drain electrode formed on the back surface of the semiconductor substrate. Japanese Patent Laid-Open No. 06-132539 discloses a structure in which the gate trench is formed in a ring-like shape.
A silicon carbide semiconductor device made of silicon carbide, such as a SiC-MOSFET, has a high breakdown voltage, so that the gate trench can be insufficient to prevent an electrical field from excessively concentrated in the gate oxide film, and the gate oxide film can break.
To avoid this, the electrical field can be prevented from being applied to a gate trench 20 by a protection trench 10 that surrounds the whole of the periphery of the gate trench 20 in the horizontal direction. However, if such a protection trench 10 is adopted, a wire of a conductive material 81, such as polysilicon, that is connected to a gate pad from above the gate trench 20 has to be placed so as to pass across the protection trench 10 (see FIG. 7). Therefore, a predetermined part of the protection trench 10 (the part indicated by an arrow in the implementation shown in FIG. 7) has to be filled with an insulating material, such as an oxide, and the wire of the conductive material has to be passed over the insulating material. Thus, the number of manufacturing steps disadvantageously increases.